Fix deformation gradient performance in 3D

[efaulhaber]

@svchb and @copilot This should be mathematically identical, but please check carefully that this is indeed identical and I did not introduce an error here.

3D

Machine	main	This PR
A4500	26.396 ms	3.457 ms (7.64x)
Intel Xeon w9-3475X (x36)	67.368 ms	39.861 ms (1.69x)
H100 FP64	5.774 ms	1.358 ms (4.25x)
H100 FP32	3.030 ms	855.816 μs (3.54x)

2D

Machine	main	This PR
A4500	402.072 μs	395.481 μs (1.02x)
Intel Xeon w9-3475X (x36)	4.900 ms	4.855 ms (1.01x)
H100 FP64	261.475 μs	256.930 μs (1.02x)
H100 FP32	157.665 μs	155.842 μs (1.01x)

[Copilot]

Pull request overview

Optimizes the Total Lagrangian SPH (TLSPH) 3D deformation gradient accumulation by algebraically reordering the tensor-product/matrix multiplication to reduce computational cost while preserving the mathematical result.

Changes:

• Rewrites the deformation gradient neighbor contribution from pos_diff * grad_kernel' * L_a' into an equivalent transposed form (L_a * grad_kernel * pos_diff')'.
• Introduces a temporary F_T (the transposed contribution) to make the faster evaluation order explicit and readable.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (6bb102a) to head (e963431).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1220      +/-   ##
==========================================
- Coverage   90.05%   90.04%   -0.01%     
==========================================
  Files         136      136              
  Lines       10594    10596       +2     
==========================================
+ Hits         9540     9541       +1     
- Misses       1054     1055       +1     

Flag	Coverage Δ
total	90.05% <100.00%> (+<0.01%)	⬆️
unit	70.77% <100.00%> (?)

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